Sequential lithographic methods to reduce stacking fault nucleation sites

ABSTRACT

An epitaxial silicon carbide layer is fabricated by forming first features in a surface of a silicon carbide substrate having an off-axis orientation toward a crystallographic direction. The first features include at least one sidewall that is orientated nonparallel (i.e., oblique or perpendicular) to the crystallographic direction. A first epitaxial silicon carbide layer is then grown on the surface of the silicon carbide substrate that includes first features therein. Second features are then formed in the first epitaxial layer. The second features include at least one sidewall that is oriented nonparallel to the crystallographic direction. A second epitaxial silicon carbide layer is then grown on the surface of the first epitaxial silicon carbide layer that includes the second features therein.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 10/929,911,filed Aug. 30, 2004, entitled Sequential Lithographic Methods to ReduceStacking Fault Nucleation Sites and Structures Having Reduced StackingFault Nucleation Sites, now U.S. Pat. No. 7,109,521, which itself claimsthe benefit of and priority from Provisional Application Ser. No.60/554,123 filed Mar. 18, 2004, entitled Lithographic Method to ReduceStacking Fault Nucleation Sites ad Reduce V_(f) Drift in Bipolar Devicesand Structures Having Reduced Stacking Fault Nucleation Sites, thedisclosures of which are hereby incorporated herein by reference intheir entirety as if set forth fully herein.

STATEMENT OF GOVERNMENT INTEREST

This invention was developed under government contract No.N00014-02-C-0302. The U.S. Government has a paid-up license in thisinvention and the right in limited circumstances to require the patentowner to license others on reasonable terms as provided for by the termsof contract No. N00014-02-C-0302 awarded by the Defense AdvancedResearch Projects Agency (“DARPA”).

FIELD OF THE INVENTION

This invention relates to semiconductor materials and devices andfabrication methods therefor, and more particularly to methods ofgrowing monocrystalline semiconductor materials, materials formedthereby and devices formed therein.

BACKGROUND

The present invention relates to methods of fabricating semiconductormaterials used in electronic devices, such as power electronic devices.In particular, some embodiments of the invention relate to processesthat can reduce crystal defects in silicon carbide, and the resultingstructures and devices. The present invention is related to the subjectmatter disclosed and claimed in co-pending and commonly assignedapplication Ser. No. 10/046,346; filed Oct. 26, 2001, now U.S. Pat. No.6,849,874, as well as co-pending and commonly assigned application Ser.No. 10/605,312 filed Sep. 22, 2003, now U.S. Pat. No. 7,018,554; thecontents of both of which are incorporated entirely herein by referencein their entirety, as set forth fully herein.

Silicon Carbide

Silicon carbide (SiC) has emerged over the last two decades as anappropriate candidate semiconductor material that can offer a number ofadvantages over both silicon and gallium arsenide. In particular,silicon carbide has a wide bandgap, a high breakdown electric field, ahigh thermal conductivity, a high saturated electron drift velocity, andis physically extremely robust. Silicon carbide has an extremely highmelting point and is one of the hardest known materials in the world.

Because of its physical properties, however, silicon carbide also may berelatively difficult to produce. Because silicon carbide can grow inmany polytypes, it may be difficult to grow into large single crystals.The high temperatures used to grow silicon carbide also may make controlof impurity levels (including doping) relatively difficult, and likewisemay raise difficulties in the production of thin films (e.g. epitaxiallayers). Because of its hardness, the traditional steps of slicing andpolishing semiconductor wafers also may be more difficult with siliconcarbide. Similarly, its resistance to chemical attack may make itdifficult to etch in a conventional fashion.

In addition, silicon carbide can form over 150 polytypes, many of whichare separated by relatively small thermodynamic differences. As aresult, growing single crystal substrates and high quality epitaxiallayers (“epilayers”) in silicon carbide has been, and may remain, adifficult task.

Nevertheless, based on a great deal of research and discovery in thisparticular field, including that carried out by the assignee of thepresent invention, a number of advances have been made in the growth ofsilicon carbide and its fabrication into useful devices. Accordingly,commercial devices are now available that incorporate silicon carbide asa substrate for other useful semiconductors such as the Group IIInitrides, to produce blue and green light emitting diodes. Also,commercial silicon carbide-based devices are available for microwave andradio frequency (RF) high-power, high-voltage applications, and/or forother applications.

As the success of silicon-carbide technology has increased theavailability of certain SiC-based devices, particular aspects of thosedevices have become more apparent. In particular, it has been observedthat the forward voltage (V_(f)) of some silicon carbide-based bipolardevices such as bipolar power devices may increase noticeably duringoperation of some devices. This increase is generally referred to as“V_(f) drift.” For a number of reasons, such functional problems insemiconductor devices can often result from defects in the crystalstructure of the material from which the devices are formed. Examples ofthese defects are discussed below.

Deposition systems and methods are commonly used to form layers ofsemiconductor materials, such as thin epitaxial films, on substrates.For example, a chemical vapor deposition (CVD) reactor system andprocess may be used to form a layer of semiconductor material such assilicon carbide (SiC) on a substrate. CVD processes may be particularlyeffective for forming layers with controlled properties, thicknesses,and/or arrangements such as epitaxial layers. Typically, in a depositionsystem, such as a CVD system, the substrate is placed in a reactionchamber within a susceptor and one or more process gases includingreagents or reactants to be deposited on the substrate are introducedinto the chamber adjacent the substrate. The process gases may be flowedthrough the reaction chamber in order to provide a uniform or controlledconcentration of the reagents or reactants to the substrate.

Silicon Carbide Crystal Structure

A deposition system, such as a CVD reactor, may be used to formepitaxial layers of silicon carbide on a single crystal silicon carbidesubstrate having a predetermined polytype such as 2H, 4H, 6H, 15R, 3Cand the like. The term “polytype” refers to the ordering and arrangementof layers of atoms within a crystal structure. Thus, although thedifferent polytypes of silicon carbide are stoichiometrically identical,they possess different crystal structures and consequently may havedifferent material properties such as bandgap, carrier mobility andbreakdown field strength. The letters H, R and C refer to the generalcrystal structure of the polytype, namely, hexagonal, rhombohedral andcubic, respectively. The numbers in the polytype designations refer tothe repetition period of layer arrangements. Thus, a 4H crystal has ahexagonal crystal structure in which the arrangement of atoms in acrystal repeats every four bi-layers.

FIG. 1 illustrates a hexagonal unit cell of a hypothetical crystal. Theunit cell 60 includes a pair of opposing hexagonal faces 61A, 61B. Thehexagonal faces are normal to the c-axis, which runs along the <0001>direction as defined by the Miller-Bravais indexing system fordesignating directions in a hexagonal crystal. Accordingly the hexagonalfaces are sometimes called the c-faces, which define the c-planes orbasal planes of the crystal. Planes perpendicular to the c-plane arereferred to as prismatic planes.

Silicon carbide possesses a number of potentially advantageous physicaland electronic characteristics for semiconductor performance anddevices. These may include a wide bandgap, high thermal conductivity,high saturated electron drift velocity, high electron mobility, superiormechanical strength, and radiation hardness. However, the presence ofcrystalline defects in silicon carbide films may limit the performanceof electronic devices fabricated in the films, depending on the type,location, and density of the defects. Accordingly, significant researchhas focused on reducing defects in silicon carbide films. Certaindefects, such as micropipes, are known to severely limit and evenprevent device performance. Other defects, such as threadingdislocations, are not considered to be individually catastrophic todevice operation, and therefore they may not significantly impact deviceperformance at densities normally found in epitaxial films.

For applications where a high voltage blocking capability is desired(for example power switching applications), silicon carbide films areusually grown “off-axis.” That is, the substrate crystal is sliced at anangle that is slightly oblique to the normal crystal axis (the c-axis).Taking for example a hexagonal polytype such as 4H or 6H, the obliqueangle of the cut may be made in one of the standard crystallographicdirections illustrated in FIG. 2, namely the <11 20> direction (towardsa point of the hexagonal unit cell) or the <10 10> direction (towardsthe center of a flat side of the hexagonal unit cell), or along adifferent direction. As a result of the off-axis slicing, the face of aprepared substrate may be characterized by a periodic arrangement ofplateaus and steps. See, for example, U.S. Pat. No. 4,912,064 to Kong etal, entitled Homoepitaxial Growth of Alpha-SiC Thin Films andSemiconductor Devices Fabricated Thereon, assigned to North CarolinaState University, the disclosure of which is hereby incorporated hereinby reference in its entirety as if set forth fully herein.

Thus when an epitaxial layer is grown on the substrate, the depositedatoms bond to atoms at the exposed edges of the crystal layer steps,which causes the steps to grow laterally in so-called step-flow fashion.Step-flow growth is illustrated in FIG. 3. Each layer or step grows inthe direction in which the crystal was originally cut off-axis (the <1120> direction in the case illustrated in FIG. 3).

Crystallographic Defects

At the most basic level, structural crystallographic defects may fallinto four categories: point defects, line defects, planar defects andthree dimensional defects. Point defects include vacancies, line defectsinclude dislocations, planar defects include stacking faults andthree-dimensional defects include polytype inclusions.

A dislocation is a kind of structural imperfection that extends for manyunit cell lengths throughout a crystal. A more definite description ofdislocation may classify them as screw and edge dislocations. Asrecognized by those persons skilled in this art, a symmetrical pathfollowed from atom to atom (or from ion to ion) in a real crystal thatreturns upon itself, is referred to as a Burgers circuit. If the samepath in the lattice that typifies the structure does not return uponitself, so that the beginning and end do not lie on the same atom, thenthe Burgers circuit encloses one or more dislocations. The vector thatcompletes the closed circuit in the lattice is referred to as theBurgers vector and measures the magnitude and direction of thedislocation.

If the Burgers vector is parallel to the line that locates thedislocation, the defect is referred to as a screw dislocation.Alternatively, if the Burgers vector is perpendicular to thedislocation, it is referred to as an edge dislocation. The simplestversion of an edge dislocation is an incomplete plane of atoms or ionsinterleaved between two normal planes in a manner somewhat analogous toan extra card inserted halfway into a deck.

Screw dislocations are not necessarily disadvantageous and, in fact, canbe advantageous for the growth of a crystal. For example, a 1C threadingscrew dislocation at the nominally (0001) oriented growth surface of asilicon carbide crystal presents a regenerating edge that is one or afew atoms high. At this edge, continued growth of the crystal isrelatively easy. Dislocations, however, allow plastic flow to occur in acrystal relatively easily. Dislocations may move preferentially alongslip planes. The dislocation moves relatively easily through the crystalbecause motion in the slipped plane involves only a slight displacementof the structural elements. Stated differently, the slipped planesprovide a low-energy intermediate state by which a crystal can bereorganized.

Defects in Silicon Carbide

In silicon carbide power devices, the availability of such a relativelylow-energy intermediate state may encourage faults to continue to grow,since the operation of the device can provide the relatively smallamount of energy that may be required to drive dislocation motion.

Commercial quality SiC wafers and epilayers typically include both screwand edge dislocations. These dislocations can be further grouped bytheir alignment within the crystal. Those dislocations that propagatealong the c-axis are called threading dislocations, while dislocationsthat lie within the c-plane are termed basal plane dislocations. Ingeneral, in SiC, it is energetically favorable that basal planedislocations may preferentially decompose into partial dislocations viathe mechanism described below:1/3<11 20>→1/3<10 10>+1/3<01 10>  Equation 1

The above decomposition reaction describes the decomposition of a basalplane dislocation into two Shockley partial dislocations. The linedefects generated during the above decomposition will bound a stackingfault defect. In fact, partial dislocations will generally bind theentire perimeter of the stacking fault unless the stacking fault reachesa free surface. This stacking fault will generally be electricallyactive in bipolar devices, and, during forward operation, the density ofthe electron-hole plasma may be reduced in the vicinity of the stackingfault. The reduced plasma density may increase the forward voltage ofthe device. A further potential complication is that throughrecombination enhanced dislocation glide, the stacking fault maycontinue to expand during forward operation of the device. This behaviorcan raise a substantial barrier to device exploitation because it canresult in devices with functional properties that can changeunpredictably during operation.

Stated differently, the application of electric current through asilicon carbide bipolar device may tend to initiate or propagate (orboth) changes in the crystal structure due to preexisting defects in thecrystal. As noted above, many SiC polytypes are in close thermodynamicproximity, and solid phase transformations are quite possible. When thestacking faults impact a significant portion of the active area of adevice, they tend to cause the forward voltage to increase in anundesirable manner that can prevent the device from operating asprecisely and/or efficiently as required or desired in manyapplications.

In some conventions, dislocation density is described by centimeters ofdislocation length per cubic centimeter of material, and thus reportdislocation density units of per square centimeter (cm⁻²). In anotherconvention (and as used herein) the off-axis orientation of 4H—SiCsubstrates for SiC epilayer growth and the common etch technique used todetect dislocations make it more convenient to use etch pit density(also in the units of cm⁻²), to describe dislocation densities in SiC.Those of skill in this art will thus recognize that for a givendislocation density expressed as cm/cm³, one could get a very differentdislocation pit density when expressed as pits/cm² depending on thetypical dislocation configuration and the off-axis angle of thesubstrate. Therefore, although the two numbers will have the same netunits (cm⁻²), they do not necessarily indicate the same actualdislocation density. For the sake of clarity and consistency, in thepresent application, dislocation density will be described as thedensity of specific pits delineated on an etched epi-surface of asilicon face prepared, 8° off-axis (0001) oriented substrate.

Present commercially available 4H—SiC substrates may have approximately1E3 to 1E5 (about 10³–10⁵) dislocations per cm² by the convention usedherein. This includes threading screw and edge dislocations and basalplane dislocations. Presumably, all types of dislocations can impactdevice performance, but the basal plane dislocation has beenparticularly implicated as the prevalent nucleation site of the stackingfaults that may cause V_(f) drift.

In turn, defects in the substrate are often replicated in epitaxiallayers grown on such substrates, thus making substrate crystal quality aprofoundly important factor with respect to the quality and performanceof resulting devices.

Conventional substrate preparation and epilayer growth practices canfairly effectively reduce the density of basal plane dislocations from1E3–1E4 cm⁻² in the substrate to about 400 cm⁻² in the epilayer. Thisreduction in dislocation density may be accomplished via changes in boththe substrate preparation and the epilayer growth operations.

Accordingly, for continued improvement in the structure and operation ofSiC-based bipolar and other devices it may be desirable to providecontinued improvement in the underlying substrates and their crystalstructures.

SUMMARY OF THE INVENTION

Epitaxial silicon carbide layers may be fabricated, according to someembodiments of the present invention, by forming a plurality of firstfeatures in a surface of a silicon carbide substrate having an off-axisorientation toward a predetermined crystallographic direction. Theplurality of first features includes at least one sidewall that isorientated nonparallel (i.e., oblique or perpendicular) to thepredetermined crystallographic direction. A first epitaxial siliconcarbide layer is then grown on the surface of the silicon carbidesubstrate that includes the plurality of first features therein. Aplurality of second features is then formed in a surface of the firstepitaxial layer. The plurality of second features includes at least onesidewall that is oriented nonparallel to the predeterminedcrystallographic direction. A second epitaxial silicon carbide layer isthen grown on the surface of the first epitaxial layer that includes theplurality of second features therein. The first and second features maybe identical or at least partially non-identical.

In some embodiments, the first and/or second features are formed bymasking the silicon carbide surface and then etching the surface that isexposed. In some embodiments, etching may be performed by dry etching.Dry etching may be performed using an etchant gas etch such as NF₃and/or SF₆. In some embodiments, NF₃ is used. In some embodiments thepredetermined crystallographic direction is the <11 20> direction.

In some embodiments, the plurality of first and/or second featuresincludes a plurality of trenches extending in a direction perpendicularto the predetermined crystallographic direction. In other embodiments,the trenches extend oblique to the predetermined crystallographicdirection. Oblique and perpendicular trenches also may be provided. Inother embodiments, the plurality of first and/or second featuresincludes a plurality of pits such as a periodically repeating pattern ofhexagonal pits. The first and second features, such as trenches, may beoffset from one another.

In some embodiments, the first epitaxial silicon carbide layer is grownon the surface of the silicon carbide substrate such that the firstepitaxial silicon carbide layer contains lower basal plane dislocationdensity than the silicon carbide substrate. Moreover, the secondepitaxial silicon carbide layer contains lower basal plane dislocationdensity that the first epitaxial silicon carbide layer in otherembodiments of the present invention.

A silicon carbide semiconductor structure according to some embodimentsof the present invention includes a silicon carbide substrate having anoff-axis orientation toward a predetermined crystallographic directionand including a plurality of first features in the surface thereof. Theplurality of first features includes at least one sidewall that isoriented nonparallel (i.e., oblique and/or perpendicular) to thepredetermined crystallographic direction. A first epitaxial siliconcarbide layer is also provided on the surface of the silicon carbidesubstrate that includes the plurality of first features therein. Secondfeatures are provided in a surface of the first epitaxial layer that isremote from the silicon carbide substrate, and a second epitaxialsilicon carbide layer is provided on the surface of the first epitaxiallayer. The predetermined crystallographic direction may be the <11 20>direction. The first and/or second features may include trenches and/ordepressions and/or may be offset as was described above. As was alsodescribed above, the first epitaxial silicon carbide layer may containlower basal plane dislocation density than the silicon carbidesubstrate, and the second epitaxial silicon carbide layer may containlower basal plane dislocation density than the first epitaxial siliconcarbide layer.

Other embodiments of the present invention fabricate an epitaxialsilicon carbide layer by epitaxially growing from at least one sidewallof a plurality of first features in a surface of the silicon carbidesubstrate having an off-axis orientation toward a predeterminedcrystallographic direction to form the epitaxial silicon carbide layer,the at least one sidewall being oriented nonparallel to thepredetermined crystallographic direction. At least one sidewall of aplurality of second features is epitaxially grown in the surface of thefirst epitaxial layer. The features may be trenches and/or depressionsas was described above.

Moreover, methods and structures according to other embodiments of thepresent invention may use monocrystalline substrates other than siliconcarbide to fabricate two or more epitaxial layers thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a hexagonal crystal unit cellstructure;

FIG. 2 is a top view of a hexagonal unit cell illustrating standardcrystallographic directions;

FIG. 3 is a schematic side view of an off-axis silicon carbide crystal;

FIG. 4 is a schematic view of an epitaxial deposition system;

FIG. 5 is a cross section of a susceptor assembly that can form a partof the deposition system of FIG. 4;

FIG. 6 is a schematic view of a SiC wafer that has been processed inaccordance with some embodiments of the invention;

FIGS. 7A and 78 are magnified views of a portion of the surface of a SiCwafer that has been processed in accordance with some embodiments of theinvention;

FIG. 8A is an interferometry-derived topographic representation of thesurface of a silicon carbide wafer that has been patterned in accordancewith some embodiments of the invention with a periodic hexagonal pitstructure;

FIG. 8B is a depth profile taken along line B—B of FIG. 8A;

FIG. 5C is a depth profile taken along line C—C of FIG. 8A;

FIG. 9A is a schematic plan view of the surface of a silicon carbidewafer that has been patterned in accordance with some embodiments of theinvention with a periodic trench structure;

FIG. 9B is a schematic elevation of the structure shown in FIG. 9A;

FIG. 9C is a schematic elevation of an alternate embodiment of thestructure shown in FIG. 9A;

FIG. 10 is a schematic elevation of a structure according to someembodiments of the invention;

FIG. 11 is a schematic elevation of a structure according to someembodiments of the invention;

FIG. 12 is a schematic elevation of a structure according to someembodiments of the invention;

FIG. 13 is a micrograph of the surface of a SiC epitaxial layeraccording to some embodiments of the invention;

FIG. 14 is a micrograph of the surface of a SiC epitaxial layeraccording to further embodiments of the invention; and

FIG. 15 is a micrograph of the surface of a SiC epitaxial layeraccording to further embodiments of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which embodiments of theinvention are shown. This invention may, however, be embodied in manydifferent forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. In thedrawings, the size and relative sizes of layers and regions may beexaggerated for clarity. It will be understood that when an element orlayer is referred to as being “on” another element or layer, it can bedirectly on the other element or layer or intervening elements or layersmay be present. In contrast, when an element is referred to as being“directly on” another element or layer, there are no interveningelements or layers present. Like numbers refer to like elementsthroughout. As used herein, the term “and/or” includes any and allcombinations of one or more of the associated listed items.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. Thus, for example, three or moreepitaxial layers may be grown according to embodiments of the inventioneven though growth of two layers is described herein.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, components, regions, layersand/or sections, these elements, components, regions, layers and/orsections should not be limited by these terms. These terms are only usedto distinguish one element, component, region, layer or section fromanother region, layer or section. Thus, a first element, component,region, layer or section discussed below could be termed a secondelement, component, region, layer or section without departing from theteachings of the present invention.

Furthermore, relative terms, such as “lower” or “bottom” and “upper” or“top,” may be used herein to describe one element's relationship toanother element as illustrated in the Figures. It will be understoodthat relative terms are intended to encompass different orientations ofthe device in addition to the orientation depicted in the Figures. Forexample, if the device in the Figures is turned over, elements describedas being on the “lower” side of other elements would then be oriented on“upper” sides of the other elements. The exemplary term “lower”, cantherefore, encompass both an orientation of “lower” and “upper,”depending on the particular orientation of the figure. Similarly, if thedevice in one of the figures is turned over, elements described as“below” or “beneath” other elements would then be oriented “above” theother elements. The exemplary terms “below” or “beneath” can, therefore,encompass both an orientation of above and below.

Embodiments of the present invention are described herein with referenceto cross-sectional and/or other views that are schematic illustrationsof idealized embodiments of the present invention. As such, variationsfrom the shapes of the illustrations as a result, for example, ofmanufacturing techniques and/or tolerances, are to be expected. Thus,embodiments of the present invention should not be construed as limitedto the particular shapes of regions illustrated herein but are toinclude deviations in shapes that result, for example, frommanufacturing. For example, a grown or deposited region illustrated as apolygon will, typically, have rounded or curved features and/or agradient of concentrations at its edges with another region rather thana discrete change from a first region to a second region of differentcomposition. Thus, the regions illustrated in the figures are schematicin nature and their shapes are not intended to illustrate the preciseshape of a region of a device and are not intended to limit the scope ofthe present invention.

According to some embodiment of the invention, methods of fabricating anepitaxial silicon carbide layer on a silicon carbide substrate having anoff-axis orientation toward a first crystallographic direction caninclude masking the first surface of the substrate with a patterned etchmask, etching the first surface of the substrate to form a plurality offeatures, each of which includes a sidewall oriented at an angle that isoblique or perpendicular to the first crystallographic direction, andgrowing an epitaxial layer of silicon carbide on the etched firstsurface. The substrate has an off-axis orientation towards a firstcrystallographic direction, such as the <11 20> direction. The partiallymasked substrate may be etched using a dry etch process such as reactiveion etching using NF₃ and/or SF₆.

In some embodiments, the plurality of features includes a plurality oftrenches extending in a direction that is oblique and/or perpendicularto the first crystallographic direction. In other embodiments, theplurality of features includes a periodically repeating pattern ofhexagonal depressions.

In some embodiments, a substrate is patterned with a plurality offeatures including sidewalls and a first epitaxial layer is grown with afirst defect density. The first epitaxial layer is then patterned with aplurality of features having sidewalls and a second epitaxial layer isgrown with a second defect density less than the first defect density.

Semiconductor structures according to some embodiments of the inventioncan include an off-axis silicon carbide substrate having a first surfaceand an epitaxial layer of silicon carbide formed on the first surface.The first surface of the substrate includes a plurality of features,each of which has a sidewall that is perpendicular or oblique to theoff-axis crystallographic direction. The epitaxial layer has a reduceddensity of basal plane dislocations compared to the substrate.

Other embodiments of the invention include an off-axis silicon carbidesubstrate having a first surface and a first epitaxial layer of siliconcarbide formed on the first surface. The surface of the substrateincludes a plurality of features, each of which has a sidewall that isperpendicular or oblique to the off-axis crystallographic direction. Thefirst epitaxial layer has a reduced density of basal plane dislocationscompared to the substrate. The first epitaxial layer further includes aplurality of features, each of which has a sidewall that isperpendicular or oblique to the off-axis crystallographic direction. Asecond epitaxial layer is grown on the first epitaxial layer. The secondepitaxial layer has a reduced density of basal plane dislocationscompared to the first epitaxial layer.

Some embodiments of the invention can provide methods of reducingstacking fault nucleation site density and/or reducing forward voltage(V_(f)) drift in silicon carbide-based bipolar devices, and siliconcarbide structures having a reduced density of basal plane dislocationsthat can serve as stacking fault nucleation sites.

Some embodiments include conducting an etch, such as a dry etch, and insome embodiments a reactive ion etch (RIE) using NF₃ and/or SF₆chemistry, on a masked surface of a silicon carbide substrate to form aplurality of structures having sidewalls from which epitaxial growth canproceed. Exemplary, but not limiting, techniques for dry etching ofsilicon carbide are set forth in U.S. Pat. Nos. 4,865,685 and 4,981,551,the contents of which are incorporated entirely herein by reference.Other techniques and chemistries for carrying out the etches describedherein are generally well-recognized in the art and will not bedescribed in detail herein other than to illustrate embodiments of theinvention.

The term “substrate” is used herein in a sense that is broad enough toinclude both a bulk single crystal (usually cut from a boule) as well asa device precursor structure, such as a wafer, that can include one ormore epitaxial layers, but which fundamentally (although not necessarilyexclusively) serves as the physical and electronic support for a deviceformed thereon.

Similarly, the terms “dry etch” and “wet etch,” are often used to referto reactive ion or plasma etches (“dry”) or to etches in molten salts orother solutions (“wet”).

Generally—but not necessarily—the substrate will be n-type, becausen-type substrates may offer a number of advantages in siliconcarbide-based devices, and may have an activated carrier concentrationof about 1E18–1E19 (1×10¹⁸–1×10¹⁹) carriers per cm³. In comparison tothe substrate, the carrier concentration of the epitaxial layer may beselected (or described) in terms of its purpose. “Conductive” layers maytypically have a carrier concentration of 1E18–19 cm⁻³. “Blocking”layers may typically have carrier concentrations less than 1E16 cm⁻³.“Active” layers may have carrier concentrations within these parameters,depending on the structure or purpose of the end device. Accordingly,although terms such as n, n+ and n− can be used to describe bothsubstrates and epilayers, such terminology should be considered in anillustrative rather than a limiting sense.

The silicon carbide substrate upon which the device is formed can betaken from a larger crystal (or “boule”), typically by the steps ofsawing a silicon carbide substrate wafer from a silicon carbide boule,and thereafter conducting a nonselective etch on the substrate wafer. Inmost circumstances, the sawed substrate wafer is lapped, polished,etched (typically RIE), and cleaned (with an acid or solvent) prior toconducting a first patterned etch according to some embodiments of theinvention. The term “lapped” is used in its typical sense; i.e., todescribe the steps of flattening the wafer surfaces using acounter-rotating lapping machine and an abrasive (e.g. diamond) slurry.Lapping can help make the wafer surfaces parallel and can reducemechanical defects such as saw markings. Similarly, the polishing,etching and cleaning steps may be otherwise carried out conventionallyprior to steps of the present invention.

Some embodiments of the invention can advantageously reduce the densityof basal plane dislocations in the epilayer. This reduction in basalplane dislocation density may be combined with the measures identifiedto isolate active device regions from the substrate and surface defectsdiscussed in previously-incorporated application Ser. No. 10/046,346,and may be used to provide SiC bipolar devices.

A deposition system 101 in which some embodiments of the presentinvention may be practiced is schematically shown in plan view in FIG.4. The deposition system 101 may be a horizontal, hot wall, flowthrough, CVD system as shown including a susceptor assembly 100, aquartz tube 180 defining a through passage 180A, an electromagneticfrequency (EMF) generator 182 (for example, including a power supply andan RF coil surrounding the tube 180) and a process gas supply 160. Aninsulative cover may be provided about the susceptor assembly 100 inaddition to or in place of the quartz tube 180. The deposition system101 may be used to form a layer or film on a substrate 120 (FIG. 5).While only a single substrate 120 is illustrated in FIG. 5, the system101 may be adapted to form films concurrently on multiple substrates120.

The substrate 120 may be a wafer or other structure formed of the sameor a different material than that of the layer to be deposited. Thesubstrate 120 may be formed of, for example, 2H—, 4H— or 6H—SiC. Thesubstrate surface upon which the film is deposited may be a basesubstrate or a first or subsequent layer superimposed on a basesubstrate. For example, the surface of the substrate 120 for receivingthe deposited film may be a layer previously deposited using thedeposition system 101 or an alternative apparatus. As will beappreciated by those of skill in the art in light of the presentdisclosure, embodiments of the present invention may be advantageouslyutilized with semiconductor materials other than those specificallymentioned herein.

Generally, the process gas supply 160 supplies a process gas into andthrough the susceptor assembly 100 as discussed below. The EMF generator182 inductively heats the susceptor assembly 100 to provide a hot zonein the susceptor assembly 100 where deposition reactions take place. Theprocess gas continues through and out of the susceptor assembly 100 asan exhaust gas that may include remaining components of the process gasas well as reaction by-products, for example. Embodiments of the presentinvention may use types of deposition systems other than hot wall CVDsystems. Other modifications to the systems and methods of the presentinvention will be apparent to those of ordinary skill in the art uponreading the description herein.

The process gas may include one or more components such as reagents,reactants, species, carriers and the like. Where it is desired to form aSiC layer on a substrate, the process gas may include precursor gasessuch as silane (SiH₄) and propane (C₃H₈) along with a carrier gas suchas purified hydrogen gas (H₂). The process gas supply 160 may beprovided from one or more pressurized containers of the gases with flowcontrol and/or metering devices as needed.

An exemplary conventional susceptor 100 is shown in FIG. 5. Thesusceptor 100 may be used, for example, in a flow through, hot wall,and/or CVD reactor, such as was illustrated in FIG. 4. The susceptor 100has a top susceptor member 100A and a bottom susceptor member 100B. Thesusceptor 100 also has a top liner 103 and a bottom liner 105 defining areaction chamber 107 therebetween. A substrate 120, such as asemiconductor wafer, is positioned in the reaction chamber 107 and maybe situated on an interior surface of a platter 154 (which may rotate),for example. A process gas P is introduced to the reaction chamber 107at one end, flowed through the reaction chamber 107 past the substrate120, and finally exhausted from the reaction chamber 107 at the oppositeend. As used herein, the term process gas refers to one or more gases.As indicated by the arrows in the reaction chamber 107 as shown in FIG.5, as the process gas flows through the reaction chamber 107 a portionof the process gas may contact the substrate 120 as intended and therebydeposit the reagents or reactants on the substrate 120 to form a layerthereon. In some systems, the reaction chamber 107 may have a length ofbetween about 0.1 and 1 meter, a width of between about 0.05 and 0.5meter, and a height of between about 1 and 10 cm. The reaction chamber107 is not limited to these dimensions, however. The susceptor membersmay include high quality graphite. Examples of CVD deposition systemsincluding susceptor designs are found in U.S. Patent Publication No. US2003/0079689 entitled Induction Heating Devices and Methods forControllably Heating an Article and U.S. patent application Ser. No.10/414,787, filed Apr. 16, 2003 entitled Methods and Apparatus forControlling Formation of Deposits in a Deposition System and DepositionsSystems and Methods Including the Same, both of which are incorporatedherein by reference in their entireties.

In certain embodiments, the susceptor members 100A, 100B are formed of amaterial suitable to generate heat responsive to eddy currents generatedtherein by the EMF generator 182, such materials and inductive heatingarrangements being well known to those of skill in the art. The membersmay be formed of graphite, and more preferably of high purity graphite.

A platter 154 or the like may be situated between the bottom member 100Band the substrate 120 to support the substrate 120. According to someembodiments, the platter 154 may be rotatively driven by a suitablemechanism (not shown). For example, the system may include a gas-drivenrotation system as described in U.S. application Ser. No. 09/756,548,entitled Gas Driven Rotation Apparatus and Method for Forming SiliconCarbide Layers, filed Jan. 8, 2001, now U.S. Pat. No. 6,569,250, and/oras described in U.S. application Ser. No. 10/117,858, entitled GasDriven Planetary Rotation Apparatus and Methods for Forming SiliconCarbide Layers, filed Apr. 8, 2002, now U.S. Pat. No. 6,797,069, thedisclosures of which are hereby incorporated herein by reference intheir entireties. Alternatively, the platter 154 may be stationary. Theplatter 154 may be adapted to hold one or multiple substrates 120. Theplatter 154 may be formed of any suitable material such as SiC coatedgraphite, solid SiC and/or solid SiC alloy. The platter 154 may beomitted such that the substrate rests on the susceptor member 100B, theliner 105, or other suitable support.

In use, the process gas supply 160 supplies a flow of the process gas Pto the reaction chamber 107 through the inlet opening 102. The processgas P flows generally in a flow direction R. As shown, some portion ofthe process gas and the reagents therein contact the substrate 120 toform the desired layer (e.g., an epilayer) on the exposed surface of thesubstrate 120.

While the foregoing deposition system 101 and methods are described asrelating to a horizontal, hot wall, CVD, flow through depositionprocess, various aspects of the present invention may be used in othertypes of deposition systems and processes. While particular embodimentshave been described with reference to “top”, “bottom” and the like,other orientations and configurations may be employed in accordance withthe invention. For example, the deposition system and process may be acold wall and/or non-horizontal flow through system and process. Thedeposition system and process may be a vapor phase epitaxy (VPE), liquidphase epitaxy (LPE), or plasma enhanced CVD (PECVD) deposition systemand process rather than a CVD system or process.

Referring now to FIG. 6, a single crystal silicon carbide substratewafer 10 on which one or more epitaxial layers is to be grown has agenerally circular perimeter. A pair of flats has been milled into thewafer to assist in wafer orientation. In particular, the wafer 10includes a primary flat 12 and a secondary flat 14. The surface 16 ofthe wafer corresponds generally to the c-face of the silicon carbidecrystal (except that the wafer has been cut at an off-axis angle αtowards the <11 20> direction. The off-axis angle α may be about 8° insome embodiments. The primary flat 12 runs along the <11 20> direction,while the secondary flat 14 is oriented along the <10 10> direction.

In some embodiments, prior to epitaxial growth, the surface 16 of thewafer 10 is masked and etched to form a predefined pattern therein.Conventional masks that comprise SiO₂, Si₃N₄, Indium Tin Oxide (ITO)and/or other conventional masking materials may be used. The masks maybe fabricated and patterned using conventional techniques. The etchprocess may include a reactive ion etch as described above. In otherembodiments, maskless etching and/or selective growth techniques may beused to form the plurality of features in the substrate. In yet otherembodiments, laser patterning may be used to form the plurality offeatures in the substrate. The pattern includes a plurality of features,which include at least one sidewall oriented at an oblique orperpendicular angle (i.e., nonparallel) to the crystallographicdirection in which step-flow epitaxial growth will occur. In theillustrated embodiment, the wafer 10 is cut off-axis towards the <11 20>direction; thus, epitaxial growth will occur in the <11 20> direction.Accordingly, in the illustrated embodiment, the wafer includes aplurality of features including at least one sidewall oriented at anoblique or perpendicular angle to the <11 20> crystallographicdirection.

An exemplary etch pattern is illustrated in FIG. 7A, which is anidealized schematic drawing of a magnified view of a portion 25 of thesurface of wafer 10 following etching. As illustrated therein, thesurface 16 of the wafer 10 includes a plurality of periodicallyrepeating hexagonal pits 20 etched therein. Each pit is surrounded bysix sidewalls 22. In some embodiments including the embodimentsillustrated in FIG. 7A, at least one pit 20, and in some embodimentseach pit 20, is oriented such that two of its sidewalls runapproximately parallel to the crystallographic direction in whichepitaxial growth will occur (in this case the <11 20> direction), whilethe remaining four sides are oblique to the crystallographic directionin which epitaxial growth will occur. In other embodiments, at least onepit 20 is oriented such that two of its sidewalls run approximatelyperpendicular to the crystallographic direction in which epitaxialgrowth will occur, while the remaining four sides are oblique to thecrystallographic direction in which epitaxial growth will occur.

A hexagonal pit 20 is illustrated in detail in FIG. 7B. The pit 20includes opposing pairs of sidewalls 22 a, 22 b, and 22 c. The pit 20 isfurther characterized by a major axis 24, which runs through the centerof the hexagon between opposing vertices and a minor axis 26, which runsthrough the center of the hexagon between opposing sides.

In some embodiments, the hexagonal pits 20 may have a depth of about 0.1to about 1 μm and a major axis of about 5 to about 30 μm. In particularembodiments, the pits 20 may have a depth of about 0.5 μm and a majoraxis of about 10 μm. In some embodiments, the pits 20 may have a ratioof major axis to depth of about 20:1. The sidewalls 22 may have a widthof about 0.25 to 5 μm. In particular embodiments, the sidewalls may havea width of about 4 μm.

FIGS. 8A–8C illustrate an exemplary structure. FIG. 8A is aninterferometry-derived topographic representation of the surface of asilicon carbide wafer that has been patterned in accordance with someembodiments of the invention with a periodic hexagonal pit structure.FIG. 8B is a depth profile taken along line B—B, while FIG. 8C is adepth profile taken along line C—C. The structure illustrated in FIGS.8A–8C has a major axis of approximately 9.73 μm, a depth ofapproximately 0.5 μm and a sidewall width of approximately 4 μm.

Other dimensions may be selected without departing from the scope of theinvention. For example, the exemplary dimensions given above assume anoff-axis angle α of about 8° and were selected based on presentunderstanding and capabilities. If a different off-axis angle is used,other dimensions may be used to increase or maximize the magnitude ofdefect reduction.

Other feature shapes besides hexagons may be used in other embodimentsof the invention. The features may for example include lines,herringbone patterns, or other polygons. FIG. 9A illustrates a portionof a substrate 30 on which a series of trenches 32 separated by mesastripes 34 have been formed, for example, by reactive ion etching. Thetrenches 32 include opposing sidewalls 32A, 32B which run perpendicularto the to the crystallographic direction in which step-flow epitaxialgrowth will occur, in this case the <11 20> direction. In someembodiments, the sidewalls are aligned oblique to the crystallographicdirection in which step-flow epitaxial growth will occur.

FIG. 9B is a side view of the substrate 30 showing the trenches 32 andthe mesas 34 according to other embodiments. In some embodiments, thetrenches 32 may have a width of about 10 to about 40 μm. In particularembodiments, the trenches 32 may have a depth of about 0.5 μm and awidth of about 18 μm. In some embodiments, the trenches 32 may have aratio of width to depth of about 36; 1. The mesas 34 may have a width ofabout 0.25 to 5 μm. In particular embodiments, the mesas 34 may have awidth of about 3 μm. A mask 36 is also illustrated in the side view ofFIG. 9B.

Sidewalls 32A, 32B may be vertical with respect to the floors 32C oftrenches 32 as illustrated in FIG. 9B or they may be angled with respectto floors 32C as illustrated in FIG. 9C. In addition, the sidewalls 32A,32B may be straight as illustrated or may comprise curved portions.Similarly, with other feature shapes such as hexagons or other polygons,the sidewalls of the feature may be vertical or angled with respect tothe substrate and may include straight or curved portions. For example,as shown in FIG. 9C, the structure may be etched such that sidewalls32A, 32B are angled with respect to the trench floors 32C. In this case,the top portions 34A of the mesas 34 are narrower than their bases. Insome embodiments, the mesas may have a width of between about 0 andabout 5 μm depending on lithographic and/or other techniques.

After the wafer has been etched to form surface features therein asdescribed above, and the mask is optionally removed, the wafer is placedinto an epitaxial growth reactor, such as reactor 100 as describedabove, and one or more epitaxial layers of silicon carbide are grown onthe wafer. As discussed above, epitaxial growth of silicon carbide on anoff-axis wafer may proceed in step-flow fashion in the direction of theoff-axis cut. Although the precise mechanism is not fully understood, itis presently believed that some basal plane dislocations propagating inthe substrate crystal are terminated at the etched sidewalls of thesurface features or are converted to a different type of dislocation andare inhibited or discouraged from propagating into the epitaxial layersas basal plane dislocations. Since some basal plane dislocations areprevented from propagating into the epitaxial layers as basal planedislocations, those dislocations may not be susceptible to decomposinginto stacking faults bounded by Shockley partial dislocations, which canform electrically active stacking faults.

Epitaxial growth from a patterned substrate is illustrated in FIG. 10.Substrate 30 includes a plurality of trenches 32 on the surface thereof.An epitaxial layer 40 is grown in step-flow fashion from the surface ofthe substrate. In accordance with some embodiments of the invention,regions 42 which nucleate and grow from the sidewalls 32A of thetrenches 32 can have reduced basal plane defect densities compared withregions 44 which do not nucleate and grow from mesa sidewalls. Inparticular, regions 42 may exhibit a reduction in basal plane defectdensity by a factor of 10.

As is shown from FIG. 10, the percentage of the epitaxial layer 40 thatmay benefit from defect reduction may depend on the geometry of thefeatures. In particular the sidewall height, width and/or spacing mayimpact the percentage. In order to provide greater reduction in basalplane defect density, it may be desirable to perform a secondfeature-forming and epitaxial layer growth cycle after growth of aninitial epitaxial layer. As illustrated in FIG. 11, substrate 30includes trenches 32 etched therein. A first epitaxial layer 40 is grownthereon which includes regions 42 having a reduced basal plane defectdensity which alternate with regions 44 that may not have reduced basalplane defect density. After growth of the first epitaxial layer 40 to apredetermined thickness, the substrate is masked and etched a secondtime to form trenches 52 which have sidewalls 52A, which in someembodiments span the regions 44. The trenches 52 may have the samedimensions as trenches 42 or at least some of the trenches 52 may havedifferent dimensions than at least some of the trenches 42. Epitaxiallayer 50 is then grown thereon. The basal plane dislocation density oflayer 50 may be reduced with respect to the basal plane dislocationdensity of layer 40. In the embodiment illustrated in FIG. 11, thetrenches 52 are formed directly above the trenches 32 formed in thesubstrate 30. As illustrated in FIG. 12, the trenches 52 formed in theepitaxial layer 40 may be offset from the trenches 32 formed in thesubstrate 30. FIG. 13 is a photograph of a portion of the surface of asilicon carbide wafer that includes mesas 54 that are offset from theunderlying mesas 34.

FIG. 14 is a Nomarski micrograph of the surface of an epitaxial layergrown on a hex-etched surface in accordance with embodiments of theinvention. FIG. 15 is a Nomarski micrograph of the surface of anepitaxial layer grown on a line-etched surface in accordance withembodiments of the invention.

After forming the first and/or second epitaxial layer, one or moreelectronic devices, such as bipolar devices, may be fabricated in thefirst and/or second epitaxial silicon carbide layers using techniquesthat are well known to those having skill in the art. The fabrication ofelectronic devices, such as bipolar devices, is well known to thosehaving skill in the art and need not be described further herein. Thesedevices are not illustrated in FIGS. 10–12 for sake of clarity.Moreover, it will be understood that three or more repetitions of thefeature-forming and epitaxial-growing steps may be performed.

The techniques referred to herein are, taken individually, generallywell-recognized and well-understood in the art and can be practicedwithout undue experimentation. Single crystal silicon carbide wafers ofthe type that may be used as starting structures herein are commerciallyavailable from Cree, Inc. 4600 Silicon Drive, Durham, N.C. 27706. Thegrowth of silicon carbide epitaxial layers can be carried out usingtechniques such as those set forth in U.S. Pat. Nos. 4,912,063;4,912,064; 5,679,153; and 6,297,522. Dry and electrolytic etching ofsilicon carbide are described in U.S. Pat. Nos. 6,034,001; 5,571,374;5,227,034; 4,981,551; and 4,865,685. The use of molten potassiumhydroxide as an etchant to identify and characterize a semiconductorsurface is well-understood, and includes versions expressed as ASTMstandards (e.g. ASTM F1404.92). Cutting, mechanical polishing andlapping of substrate wafers are also entirely conventional in this art.

In the drawings and specification, there have been disclosed typicalembodiments of the invention, and, although specific terms have beenemployed, they have been used in a generic and descriptive sense onlyand not for purposes of limitation, the scope of the invention being setforth in the following claims.

1. A method of fabricating an epitaxial silicon carbide layercomprising: forming a plurality of first features in a surface of asilicon carbide substrate having an off-axis orientation toward apredetermined crystallographic direction, the plurality of firstfeatures including at least one first sidewall that is orientednonparallel to the predetermined crystallographic direction; growing afirst epitaxial silicon carbide layer on the surface of the siliconcarbide substrate that includes the plurality of first features therein,the first epitaxial silicon carbide layer being thicker than a depth ofthe at least one first sidewall; forming a plurality of second featuresin a surface of the first epitaxial layer, the plurality of secondfeatures including at least one second sidewall that is orientednonparallel to the predetermined crystallographic direction; and growinga second epitaxial silicon carbide layer on the surface of the firstepitaxial layer that includes the plurality of second features therein,the second epitaxial silicon carbide layer being thicker than a depth ofthe at least one second sidewall.
 2. A method according to claim 1wherein forming a plurality of first features comprises: masking thesurface of the silicon carbide substrate having an off-axis orientationtoward a predetermined crystallographic direction; and etching thesurface of the silicon carbide substrate that is exposed by the maskingto form the plurality of first features in the surface of a siliconcarbide substrate that is masked.
 3. A method according to claim 2wherein etching comprises dry etching the surface of the silicon carbidesubstrate that is masked using NE₃ and/or SF₆.
 4. A method according toclaim 1 wherein the predetermined crystallographic direction is a <1120> direction.
 5. A method according to claim 1 wherein forming aplurality of first features comprises forming a plurality of trenches inthe surface of the silicon carbide substrate having an off-axisorientation toward a predetermined crystallographic direction, theplurality of trenches extending oblique and/or perpendicular to thepredetermined crystallographic direction and including at least onefirst sidewall that is oriented nonparallel to the predeterminedcrystallographic direction.
 6. A method according to claim 1 whereinforming a plurality of first features comprises forming a plurality ofdepressions in the surface of the silicon carbide substrate having anoff-axis orientation toward a predetermined crystallographic directionand including at least one first sidewall that is oriented nonparallelto the predetermined crystallographic direction.
 7. A method accordingto claim 6 wherein the plurality of depressions comprises a periodicallyrepeating pattern of hexagonal pits.
 8. A method according to claim 1wherein forming a plurality of second features comprises: masking thesurface of the first epitaxial layer; and etching the surface of thefirst epitaxial layer that is exposed by the masking to form theplurality of second features in the first epitaxial layer that ismasked.
 9. A method according to claim 8 wherein etching comprises dryetching the first epitaxial layer that is masked using NF₃ and/or SF₆.10. A method according to claim 1 wherein forming a plurality of secondfeatures comprises forming a plurality of trenches in the surface of thefirst epitaxial layer, the plurality of trenches extending obliqueand/or perpendicular to the predetermined crystallographic direction andincluding at least one second sidewall that is oriented nonparallel tothe predetermined crystallographic direction.
 11. A method according toclaim 1 wherein forming a plurality of second features comprises forminga plurality of depressions in the surface of the first epitaxial layerand including at least one second sidewall that is oriented nonparallelto the predetermined crystallographic direction.
 12. A method accordingto claim 11 wherein the plurality of depressions comprises aperiodically repeating pattern of hexagonal pits.
 13. A method accordingto claim 1: wherein growing the first epitaxial silicon carbide layercomprises growing the first epitaxial silicon carbide layer on thesurface of the silicon carbide substrate that includes the plurality offirst features therein such that the first epitaxial silicon carbidelayer contains lower basal plane dislocation density than the siliconcarbide substrate; and wherein growing the second epitaxial siliconcarbide layer comprises growing the second epitaxial silicon carbidelayer on the surface of the first epitaxial silicon carbide layer thatincludes the plurality of second features therein such that the secondepitaxial silicon carbide layer contains lower basal plane dislocationdensity than the first epitaxial silicon carbide layer.
 14. A methodaccording to claim 1 wherein the first and second features are laterallyoffset from one another.
 15. A method according to claim 1 furthercomprising: forming a bipolar device in the second epitaxial siliconcarbide layer.
 16. A method of fabricating an epitaxial monocrystallinelayer comprising: forming a plurality of first features in a surface ofa monocrystalline substrate having an off-axis orientation toward apredetermined crystallographic direction, the plurality of firstfeatures including at least one first sidewall; growing a firstepitaxial layer on the surface of the monocrystalline substrate thatincludes the plurality of features therein, the first epitaxial layerbeing thicker than a depth of the at least one first sidewall; forming aplurality of second features in a surface of the first epitaxial layer,the plurality of second features including at least one second sidewall;and growing a second epitaxial layer on the surface of the firstepitaxial layer that includes the plurality of second features therein,the second epitaxial layer being thicker than a depth of the at leastone second sidewall.
 17. A method according to claim 16 wherein forminga plurality of first features comprises: masking the surface of themonocrystalline substrate having an off-axis orientation toward apredetermined crystallographic direction; and etching the surface of themonocrystalline substrate that is masked to form the plurality of firstfeatures in the surface of a monocrystalline substrate that is masked.18. A method according to claim 16 wherein forming a plurality of secondfeatures comprises: masking the surface of the first epitaxial layer;and etching the surface of the first epitaxial layer that is masked toform the plurality of second features in the surface of a firstepitaxial layer that is masked.
 19. A method according to claim 16:wherein growing the first epitaxial layer comprises growing the firstepitaxial layer on the surface of the monocrystalline substrate thatincludes the plurality of first features therein such that the firstepitaxial layer contains lower basal plane dislocation density than themonocrystalline substrate; and wherein growing the second epitaxiallayer comprises growing the second carbide layer on the surface of thefirst layer that includes the plurality of second features therein suchthat the second epitaxial layer contains lower basal plane dislocationdensity than the first epitaxial layer.
 20. A method according to claim16 wherein the monocrystalline substrate comprises silicon carbide. 21.A method according to claim 16 wherein the first and second features arelaterally offset from one another.
 22. A method of fabricating anepitaxial silicon carbide layer comprising: epitaxially growing from atleast one first sidewall of a plurality of first features in a surfaceof a silicon carbide substrate having an off-axis orientation toward apredetermined crystallographic direction to form a first epitaxialsilicon carbide layer that is thicker than a depth of the at least onefirst sidewall, the at least one first sidewall being orientednonparallel to the predetermined crystallographic direction; andepitaxially growing at least one second sidewall of a plurality ofsecond features in a surface of the first epitaxial layer to form asecond epitaxial silicon carbide layer that is thicker than a depth ofthe at least one second sidewall, the at least one second sidewall beingoriented nonparallel to the predetermined crystallographic direction.23. A method according to claim 22 wherein the predeterminedcrystallographic direction is a <11 20> direction.
 24. A methodaccording to claim 22 wherein the plurality of first features comprisesa plurality of trenches in the surface of the silicon carbide substratehaving an off-axis orientation toward a predetermined crystallographicdirection, the plurality of trenches extending perpendicular to thepredetermined crystallographic direction and including at least onefirst sidewall that is oriented nonparallel to the predeterminedcrystallographic direction.
 25. A method according to claim 22 whereinthe plurality of first features comprises a plurality of depressions inthe surface of the silicon carbide substrate having an off-axisorientation toward a predetermined crystallographic direction, theplurality of depressions including at least one first sidewall that isoriented nonparallel to the predetermined crystallographic direction.26. A method according to claim 25 wherein the plurality of depressionscomprises a periodically repeating pattern of hexagonal pits.
 27. Amethod according to claim 22 wherein the plurality of second featurescomprises a plurality of trenches in the surface of the first epitaxiallayer, the plurality of trenches extending perpendicular to thepredetermined crystallographic direction and including at least onesecond sidewall that is oriented nonparallel to the predeterminedcrystallographic direction.
 28. A method according to claim 22 whereinthe plurality of second features comprises a plurality of depressions inthe surface of the first epitaxial layer, the plurality of depressionsincluding at least one second sidewall that is oriented nonparallel tothe predetermined crystallographic direction.
 29. A method according toclaim 28 wherein the plurality of depressions comprises a periodicallyrepeating pattern of hexagonal pits.
 30. A method according to claim 22wherein the epitaxial silicon carbide layer contains lower basal planedislocation density than the silicon carbide substrate.
 31. A methodaccording to claim 22: wherein the first epitaxial silicon carbide layercontains lower basal plane dislocation density than the silicon carbidesubstrate; and wherein the second epitaxial silicon carbide layercontains lower basal plane dislocation density than the first epitaxialsilicon carbide layer.
 32. A method according to claim 22 wherein thefirst and second features are laterally offset from one another.